Anatomical evidence for glutamate and/or aspartate as neurotransmitters in the geniculo-, claustro-, and cortico-cortical pathways to the cat striate cortex

Bioanalytical methodology for determination of glutamate and aspartate for use in pharmacological sciences with application of Integrated Calibration Method

A new calibration approach based on the adaptation of Integrated Calibration Method (ICM) to consecutive two components analysis in separation science is presented. Consecutive ICM method (C-ICM) was conceptually developed and applied to determination of two excitatory amino acids - glutamate and aspartate in cerebrospinal fluids collected by the use of brain microdialysis from freely-moving animals. Both analytes as a neurotransmitters play an important role in formation of the memory trace, and thus the processes of learning and memory. Due to their low concentration and presence of interferences, considered analytical system - animal brain - was a big challenge. High-performance liquid chromatography (HPLC) with electrochemical detection (ECD) was used in all experimental work. The most important feature of proposed method is integration of interpolative and extrapolative ways to calculate analyte concentration in single calibration procedure, which consequently leads to obtain series of six estimations of analytical result. Comparison of individual estimations with each other allows for a more in-depth analysis of systematic errors. It was proved that C-ICM approach enables diagnosis and compensation of systematic errors induced by occurrence of interference effects and improvement of accuracy of analytical results. Most of all, it was demonstrated that application of this method is efficient and useful analytical tool in analysis of complicated biological samples in pharmacology and neuroscience.

Silencing "Top-Down" Cortical Signals Affects Spike-Responses of Neurons in Cat's "Intermediate" Visual Cortex

We examined the effects of reversible inactivation of a higher-order, pattern/form-processing, postero-temporal visual (PTV) cortex on the background activities and spike-responses of single neurons in the ipsilateral cytoarchitectonic area 19 (putative area V3) of anesthetized domestic cats. Very occasionally (2/28), silencing recurrent “feedback” signals from PTV, resulted in significant and reversible reduction in background activity of area 19 neurons. By contrast, in large proportions of area 19 neurons, PTV inactivation resulted in: (i) significant reversible changes in the peak magnitude of their responses to visual stimuli (35.5%; 10/28); (ii) substantial reversible changes in direction selectivity indices (DSIs; 43%; 12/28); and (iii) reversible, upward shifts in preferred stimulus velocities (37%; 7/19). Substantial (≥20°) shifts in preferred orientation and/or substantial (≥20°) changes in width of orientation-tuning curves of area 19 neurons were however less common (26.5%; 4/15). In a series of experiments conducted earlier, inactivation of PTV also induced upward shifts in the preferred velocities of the ipsilateral cytoarchitectonic area 17 (V1) neurons responding optimally at low velocities. These upward shifts in preferred velocities of areas 19 and 17 neurons were often accompanied by substantial increases in DSIs. Thus, in both the primary visual cortex and the “intermediate” visual cortex (area 19), feedback from PTV plays a modulatory role in relation to stimulus velocity preferences and/or direction selectivity, that is, the properties which are usually believed to be determined by the inputs from the dorsal thalamus and/or feedforward inputs from the primary visual cortices. The apparent specialization of area 19 for processing information about stationary/slowly moving visual stimuli is at least partially determined, by the feedback from the higher-order pattern-processing visual area. Overall, the recurrent signals from the higher-order, pattern/form-processing visual cortex appear to play an important role in determining the magnitude of spike-responses and some “motion-related” receptive field properties of a substantial proportion of neurons in the intermediate form-processing visual area—area 19.

Claustrum: a case for directional, excitatory, intrinsic connectivity in the rat

Claustrum, a gray matter structure that underlies the neocortex, is reciprocally connected with many neocortical and limbic cortical areas. This connectivity positions claustrum ideally for the integration or coordination of widespread cortical activity. In anatomical studies using multiple planes of section, claustrum has distinct subregions based on latexin immunohistochemistry, and an approximately rostro-caudal alignment of fusiform cells supporting a laminar intrinsic organization. Physiological studies of claustral connectivity in disinhibited brain slices demonstrate (1) intrinsic connectivity sufficient to generate spontaneous synchronized burst discharges, (2) activity spread within the oblique laminae that contained the principal cellular axis, and (3) segregation of activity as evidenced by the absence of spread within coronal planes. Activity spread depended on glutamatergic synaptic transmission, and activity restrictions did not depend on inhibitory circuits. We conclude that the claustrum has an intrinsic excitatory connectivity that is constrained in approximately rostro-caudal laminae, with minimal cross-communication between laminae. Further, claustrum has the intrinsic capability of generating synchronized population activity and facilitating its spread within laminae, a feature that may contribute to seizure generation and spread.

Comparative organization of the claustrum: what does structure tell us about function?

The claustrum is a subcortical nucleus present in all placental mammals. Many anatomical studies have shown that its inputs are predominantly from the cerebral cortex and its outputs are back to the cortex. This connectivity thus suggests that the claustrum serves to amplify or facilitate information processing in the cerebral cortex. The size and the complexity of the cerebral cortex varies dramatically across species. Some species have lissencephalic brains, with few cortical areas, while others have a greatly expanded cortex and many cortical areas. This evolutionary diversity in the cerebral cortex raises several questions about the claustrum. Does its volume expand in coordination with the expansion of cortex and does it acquire new functions related to the new cortical functions? Here we survey the organization of the claustrum in animals with large brains, including great apes and cetaceans. Our data suggest that the claustrum is not always a continuous structure. In monkeys and gorillas there are a few isolated islands of cells near the main body of the nucleus. In cetaceans, however, there are many isolated cell islands. These data suggest constraints on the possible function of the claustrum. Some authors propose that the claustrum has a more global role in perception or consciousness that requires intraclaustral integration of information. These theories postulate mechanisms like gap junctions between claustral cells or a “syncytium” to mediate intraclaustral processing. The presence of discontinuities in the structure of the claustrum, present but minimal in some primates, but dramatically clear in cetaceans, argues against the proposed mechanisms of intraclaustral processing of information. The best interpretation of function, then, is that each functional subdivision of the claustrum simply contributes to the function of its cortical partner.

Cardiovascular responses of the anterior claustrum; its mechanism; contribution of medial prefrontal cortex

The anterior claustrum (CLa) has bilateral connections with the areas involved in cardiovascular regulation, though its role in cardiovascular control is not yet understood. This study was performed to find the cardiovascular responsive region of the CLa by stimulating all parts of the CLa with l-glutamate, and to find the possible mechanisms mediating its responses in urethane-anesthetized rats. We also investigated the possible involvement of the medial prefrontal cortex in the cardiovascular responses of the CLa. The effect of microinjection of l-glutamate (50-100 nl, 0.25 M) was tested throughout the Cla and only in one area at 2.7 mm rostral to bregma, 1.8-2.0 midline and 4.5-5.6mm vertical, significant decreases in arterial pressure were elicited (-21.71±2.1 mmHg, P<0.001, t-test) with no significant change in heart rate. Administration (i.v.) of the muscarinic receptor blocker, atropine, had no effect on the change in mean arterial pressure in response to glutamate stimulation, suggesting that the parasympathetic system was not involved in this response. However, administration (i.v.) of the nicotinic receptor blocker, hexamethonium dichloride abolished the depressor response to glutamate, suggesting that CLa stimulation decreases sympathetic outflow to the cardiovascular system. In addition, microinjection of the reversible synaptic blocker, cobalt chloride, into the medial prefrontal cortex greatly attenuated the depressor response elicited by microinjection of glut into the CLa. Thus for the first time, we found the cardiovascular responsive region of the anterior claustrum. Also we showed that its response is mediated through the medial prefrontal cortex.

Quantitative mapping of the local and extrinsic sources of GABA and Reelin to the layer Ia neuropil in the adult rat neocortex

Inputs to apical dendritic tufts have been considered to be crucial for associative learning, attention and similar ''feedback'' interactions and are located in neocortical layer Ia. Excitatory thalamic projections to apical tufts in layer Ia have been well characterized and their role in the cortical circuit has been emphasized. In addition, the neuropil and the extracellular matrix surrounding apical tufts are highly reactive to GABA and to the glycoprotein Reelin, respectively. Recently it has been shown that the GABA inhibition on apical dendrites can reduce the output of pyramidal cells in layer V, however, the origin of 89% of the symmetric synapses in layer I still remains unknown. In the present study we have systematically analyzed the origin of the GABAergic neuropil in neocortical layer Ia in a qualitative and quantitative manner, and investigated the possible extrinsic origin of the rich extracellular Reelin content of the same layer. We show that the inhibitory inputs in a given spot in layer I come from cortical projections and arise mainly from Martinotti cells located directly under that same spot. Double bouquet and bipolar cells may also project to layer Ia although to a lesser extent and the external globus pallidus and zona incerta provide the remaining inhibitory inputs. Finally, our results suggest that Martinotti cells are also the main source of Reelin in layer Ia. The present data will help in the understanding of the cortical circuit and why it changes in pathological conditions.

Influence of 'feedback' signals on spatial integration in receptive fields of cat area 17 neurons

'Feedback' signals from mammalian extrastriate visual cortices are reported to exert primarily an excitatory influence on the classical receptive field (CRF) of neurons in the primary visual cortex (V1). However, given the much larger CRFs of neurons in extrastriate visual cortices it is not yet understood how feedback signals influence the spatial integration of visual signals by V1 neurons. To investigate this, we reversibly inactivated one of the 'form-processing' extrastriate visual cortices, the postero-temporal visual (PTV) cortex, and examined changes in responses of V1 neurons to drifting grating patches up to 28 degrees in diameter. We found that during inactivation of PTV cortex the magnitude of the responses to CRF-confined stimuli and that to large stimuli inducing maximum suppression (i.e. minimum responses) was significantly reduced, while the spatial extent of the CRF remained largely unaffected. As a result, the relative strength of the surround suppression increased marginally. This effect was apparent in both simple and complex cells. It was also strong and consistent in cells located in supragranular and infragranular layers. For those cells exhibiting some relief from surround suppression or 'counter-suppression' when large stimuli patches were applied, the effect on counter-suppression was heterogeneous. Overall, the relative integrated responses to the 28 degrees grating patches were also decreased when PTV cortex was inactivated. Thus, a substantial reduction in the CRF response and the largely unaffected spatial extent of the CRF as well as a weak surround effect observed in the present study are consistent with a multiplicative scaling effect.

'Top-down' influences of ipsilateral or contralateral postero-temporal visual cortices on the extra-classical receptive fields of neurons in cat's striate cortex

In anesthetized and immobilized domestic cats, we have studied the effects of brief reversible inactivation (by cooling to 10 degrees C) of the ipsilateral or contralateral postero-temporal visual (PTV) cortices on: 1) the magnitude of spike-responses of neurons in striate cortex (cytoarchitectonic area 17, area V1) to optimized sine-wave modulated contrast-luminosity gratings confined to the classical receptive fields (CRFs) and 2) the relative strengths of modulation of CRF-induced spike-responses by gratings extending into the extra-classical receptive field (ECRF). Consistent with our previous reports (Bardy et al., 2006; Huang et al., 2007), inactivation of ipsilateral PTV cortex (presumed homologue of primate infero-temporal cortex) resulted in significant reversible changes (almost all substantial reductions) in the magnitude of spike-responses to CRF-confined stimuli in about half of the V1 neurones. Similarly, in half of the present sample, inactivation of ipsilateral PTV cortex resulted in significant reversible changes (in over 70% of cases, reduction) in the relative strength of ECRF modulation of the CRF-induced spike-responses. By contrast, despite the fact that receptive fields of all V1 cells tested were located within 5 degrees of representation of the zero vertical meridian, inactivation of contralateral PTV cortex only rarely resulted in significant (yet invariably small) changes in the magnitude of spike-responses to CRF-confined stimuli or significant (again invariably small) changes in the relative strength of ECRF modulation of spike-responses. Thus, the ipsilateral, but not contralateral, 'higher-order' visual cortical areas make significant contribution not only to the magnitude of CRF-induced spike-responses but also to the relative strengths of ECRF-induced modulation of the spike-responses of V1 neurons. Therefore, the feedback signals originating from the ipsilateral higher-order cortical areas appear to make an important contribution to contextual modulation of responses of neurons in the primary visual cortices.

Neurochemically defined cell types in the claustrum of the cat

The claustrum is a subcortical structure reciprocally and topographically connected with all sensory and motor domains of the cerebral cortex. Previous anatomical and electrophysiological data suggested that most cells in the claustrum are large neurons that both receive cortical input and project back to cortex, forming excitatory connections with their cortical targets. These data have been interpreted to imply a relay function for the claustrum, with information from different functional cortical domains remaining segregated. The possibility that the claustrum might mediate a more "global" function has been recently been developed by Crick and Koch [Crick, F. C., Koch, C., 2005. What is the function of the claustrum? Philos. Trans. R. Soc. Lond., B Biol. Sci. 360, 1271-1279]. We have reexamined the anatomical substrate for information processing in the claustrum of the cat by analyzing the patterns of immunoreactivity to calcium-binding proteins, GAD, serotonin, nNOS and the glutamate transporter EAAC1. We found multiple neurochemically defined cell types, suggesting multiple classes of projection neurons and interneurons. Each class was found throughout the entire claustrum, in all functionally defined subdivisions. Many neurons in the claustrum were surrounded by parvalbumin, calretinin, GAD or nNOS immunoreactive terminals, suggesting that many neurons of the claustrum make extensive intraclaustral connections. The entire claustrum also receives a serotonergic input. The identification of multiple neurochemical cell classes, their distribution and the extent of their dendritic arborizations relative to functional compartments suggest a substrate for information processing in the claustrum that may allow integration of information across functional subdivisions.

The effects of reversible inactivation of postero-temporal visual cortex on neuronal activities in cat's area 17

'Spontaneous' and visually evoked action potentials were recorded from single neurons in cytoarchitectonic area 17 (striate cortex, area V1) of anaesthetized and immobilized cats, prior to, during and after brief reversible inactivation of the ipsilateral postero-temporal visual (PTV) cortex (presumed homologue of primate inferotemporal cortex). Inactivation of PTV cortex resulted: 1) in significant changes in the response magnitude (mostly a reduction) to optimal and/or sub-optimal visual stimuli in over 55% of area 17 cells and 2) significant changes (usually a reduction) in the 'spontaneous' (background) activity of about two-thirds of the cells in which inactivation of PTV cortex significantly affected the magnitude of responses to optimal stimuli. In over 85% of the significantly affected area 17 cells, rewarming PTV cortex to normal temperature (36 degrees C) resulted in the recovery of both the magnitude of responses and the background activity to levels not significantly different from pre-inactivation levels. Irrespective of the significance of changes in the magnitude of responses, in a substantial proportion of area 17 cells, inactivation of PTV cortex resulted in changes in some receptive field characteristics. Thus, there were substantial (20% or more) changes in orientation tuning widths (in over a quarter of the sample) and/or direction selectivity indices (in about a third of the sample). Thus, the feedback signals originating from PTV cortex, like signals originating from some other 'higher-order' visual cortical areas exert a clear modulatory influence on the responsiveness, background activity and some receptive field properties of neurons in the ipsilateral area 17.

Differentiation in the immunocytochemical features of intrinsic and cortically projecting neurons in the rat claustrum -- combined immunocytochemical and axonal transport study

Retrograde axonal transport method of the fluorescent tracer FluoroGold (FG) was combined with immunocytochemistry to investigate the occurrence of nitric oxide synthase (NOS), somatostatin (SOM), neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP) in both intrinsic and cortically projecting neurons of the rat claustrum. Only NOS was detected in both the scattered projecting neurons and internal neurons of the claustrum. Approximately 20% of NOS-immunoreactive neurons in the claustrum were also retrogradely labeled with FG after tracer injections into the frontal cortex. The other substances were exclusively confined to the population of interneurons, which mainly displayed an oval, round or fusiform shape and a medium size. Apart from the neuronal somata, the proximal parts of the dendritic arborization were clearly visible. The immunoreactive neurons were randomly distributed in the claustrum and their neuronal size and shape did not differ in the various parts of the studied structure. Co-localization of NOS and SOM or NOS and NPY was reported. In conclusion, SOM, VIP and NPY do not appear to play a significant role in the claustro-cortical projection but are most probably involved in modulation and information transfer in the claustrum. The appearance of NOS in both cortically projecting and intrinsic neurons of the claustrum may be indicative of a fundamentally different role in the functioning of the claustro-cortical loop.

Effects of feedback projections from area 18 layers 2/3 to area 17 layers 2/3 in the cat visual cortex

In the absence of a direct geniculate input, area 17 cells in the cat are nevertheless able to respond to visual stimuli because of feedback connections from area 18. Anatomic studies have shown that, in the cat visual cortex, layer 5 of area 18 projects to layer 5 of area 17, and layers 2/3 of area 18 project to layers 2/3 of area 17. What is the specific role of these connections? Previous studies have examined the effect of area 18 layer 5 blockade on cells in area 17 layer 5. Here we examine whether the feedback connections from layers 2/3 of area 18 influence the orientation tuning and velocity tuning of cells in layers 2/3 of area 17. Experiments were carried out in anesthetized and paralyzed cats. We blocked reversibly a small region (300 microm radius) in layers 2/3 of area 18 by iontophoretic application of GABA and recorded simultaneously from cells in layers 2/3 of area 17 while stimulating with oriented sweeping bars. Area 17 cells showed either enhanced or suppressed visual responses to sweeping bars of various orientations and velocities during area 18 blockade. For most area 17 cells, orientation bandwidths remained unaltered, and we never observed visual responses during blockade that were absent completely in the preblockade condition. This suggests that area 18 layers 2/3 modulate visual responses in area 17 layers 2/3 without fundamentally altering their specificity.